1. Field of the Invention
The present invention relates to an antenna coil for mobile communication and an antenna device including the antenna coil.
2. Description of the Related Art
In recent years, there has been an increased use of contactless communication in which an electromagnetic induction method is used. In such contactless communication, a portable electronic device, such as a mobile terminal, and a reader/writer each have an antenna and exchange data with each other. An antenna included in the portable electronic device must be highly sophisticated, low priced, and miniaturized. In order to meet these requirements, antenna coils are typically used.
Japanese Unexamined Patent Application Publication No. 2003-22912 discloses an antenna coil in which a flexible substrate including a coil conductor disposed thereon is mounted on a magnetic core. FIG. 11 is a perspective view illustrating a configuration of an antenna coil disclosed in Japanese Unexamined Patent Application Publication No. 2003-22912. A single flexible substrate 503 including a coil conductor disposed thereon is mounted on a magnetic core 502. FIG. 12 is a plan view illustrating a main surface of the flexible substrate 503 before the flexible substrate 503 is mounted on the magnetic core 502. In Japanese Unexamined Patent Application Publication No. 2003-22912, the flexible substrate 503 is bent to produce a plurality of surfaces. The surfaces of the flexible substrate 503 are individually bonded to the main surface and side surfaces of the magnetic core 502. Thus, by mounting the flexible substrate 503 on the magnetic core 502, a first coil portion 504a and a second coil portion 504b are provided on the magnetic core 502. Here, the coil conductor is defined by a line provided on the flexible substrate, and the coil portion is defined by a spiral portion of the coil conductor.
The antenna coil disclosed in Japanese Unexamined Patent Application Publication No. 2003-22912 is usually provided on a circuit substrate, such that the main surface of the antenna coil is adjacent to the main surface of the circuit substrate. When an antenna device including such an antenna coil is used in a mobile terminal, a user of the mobile terminal holds the mobile terminal over a reader/writer such that the main surface of the mobile terminal, which is parallel or substantially parallel to the circuit substrate therein, overlaps the main surface of the reader/writer in plan view.
However, in a mobile terminal including the antenna coil disclosed in Japanese Unexamined Patent Application Publication No. 2003-22912, if the mobile terminal is held over a reader/writer as described above, it may not be able to communicate with the reader/writer or a communication range between the mobile terminal and the reader/writer may be reduced. That is, an antenna coil 501 disclosed in Japanese Unexamined Patent Application Publication No. 2003-22912 can be interlinked with a magnetic flux parallel or substantially parallel to the main surface of the antenna coil 501, but cannot be interlinked with a magnetic flux perpendicular or substantially perpendicular to the main surface of the antenna coil 501.
There are two reasons for this. The first reason is that a coil conductor is provided on substantially the entire main surface of the antenna coil 501. As a result, a magnetic flux with which the antenna coil 501 can be interlinked is limited. The second reason is that the winding directions of the first coil portion 504a and the second coil portion 504b with respect to the coil axis are the same. As a result, even if a magnetic flux enters the main surface of the antenna coil 501, at the time the magnetic flux passes through the coil axis of the first coil portion 504a and the second coil portion 504b, the directions of magnetic fluxes passing through the first coil portion 504a and the second coil portion 504b are opposite to one another. Accordingly, voltages are induced in opposite directions, and the induced voltages substantially cancel each other. This makes it difficult to maintain an optimum communication state.